During operation of a continuous ink jet printer, a continuous stream of ink drops is generated and means are provided for deflecting the drops in flight, so that different drops can travel to different destinations. Since the drops are generated continuously, only some of the drops will be required for printing. Accordingly, the drops required for printing are arranged to travel in a direction so that they reach the surface to be printed onto, whereas drops which are not required for printing are arranged to travel to a means, usually known as a gutter, where they are collected. In almost all modern continuous ink jet printers, ink collected at the gutter is returned to an ink tank, from which ink is supplied to the means (sometimes called the ink gun) which creates the stream of ink drops. Ink jet printers of this type are used for a wide variety of printing and marking purposes, such as printing “sell by” and batch information on food containers and printing identification and other variable data on industrial products and packaging.
Typically, the ink is electrically conductive when wet, and an arrangement of electrodes is provided to trap electric charges on the ink drops and create electrostatic fields in order to deflect the charged drops. The ink gun, the various electrodes and the gutter are fixed in the appropriate spatial relationship in a printhead. Various tanks, pumps, control circuits and the like are housed within a printer body, and the head is usually connected to the body by a flexible conduit carrying fluid lines and electrical wiring, which may be a few meters long.
The ink contains one or more colouring substances together with various other components, carried in a solvent such as methylethylketone or, in the case of inks for food use, ethanol. The solvent is highly volatile, to ensure that the printed ink drops dry quickly. Consequently, the solvent has a tendency to evaporate from the ink during operation of the printer, so that the ink in the ink tank becomes too concentrated. Accordingly, a typical ink jet printer will also have a tank of spare solvent, also housed in the main body, and an arrangement for monitoring ink viscosity directly or indirectly. When the viscosity exceeds a predetermined level, a small dose of solvent will be transferred from the solvent tank into the ink tank to dilute the ink.
In order that the ink collected by the gutter should be conveyed along the gutter, line away from the gutter, suction is usually applied to the gutter line from a suction source, typically in the main printer body. The fluid travelling along the gutter line will be a mixture of ink and air. Air inevitably enters the gutter both as a result of the suction applied to the gutter line and because the ink drops moving through the air from the ink gun to the gutter inevitably entrain some air in their path. This mixture of ink and air is delivered to the ink tank.
In order to maintain the ink and solvent tanks at the correct pressure, they may both be vented to allow air to flow in and out of the tanks. Each tank may be vented independently, or alternatively the ink tank may be vented to the solvent tank and the solvent tank may be vented to atmosphere. The air which enters the ink tank with the ink recovered from the gutter is therefore able to escape through the venting arrangement.
Even in the case of printers in which the ink and solvent tanks are pressurised, such as the arrangement of DE-A-3607237, an arrangement must be provided for venting the air which has entered through the gutter.
It is also known to deliver the mixture of ink and air from the gutter to a settling tank, rather than directly to the ink tank, to allow the ink and air to separate before the ink is returned to the ink tank. This can be useful in cases where the ink tends to foam or there is a tendency for very small air bubbles to be mixed into the ink. In this case, the air which has entered through the gutter may be vented from the settling tank without passing through the ink tank.
In the operation of a continuous ink jet printer the loss of solvent through evaporation takes place almost entirely through the air which enters the gutter, because the intimate contact of that air with the ink in the gutter line means that the air tends to be highly laden with solvent vapour when it is discharged to atmosphere.
U.S. Pat. No. 4,023,182 proposes a tank, to allow the air and ink to separate, connected to the gutter by a short tube of relatively large diameter. The air is discharged from the tank through another large diameter tube to a vacuum source which is principally responsible for the suction applied to the gutter. The ink is transferred separately through a relatively narrow diameter tube to an evacuated ink return tank. This arrangement is intended to minimise the extent to which the air and ink can mix before they are separated in the tank, so as to reduce the amount of solvent that evaporates from the ink.
WO02/100645 proposes an arrangement for minimising the formation of an ink-air foam or emulsion in the gutter line, in order to avoid the build-up of such a foam or emulsion in the ink tank. It provides a gutter specially shaped to allow drops to form a liquid film and then a pool of ink with little splashing of the drops on impact. The build-up of the ink pool at the gutter is monitored and suction is applied to the gutter line only when there is ink to be evacuated. This arrangement reduces the extent to which the ink and the air mix, and also reduces the total amount of air sucked through the gutter line. It mentions controlling the manner of switching suction to the gutter line in order to minimise consumption of solvent.
WO99/62717 proposes to apply only an intermittent or pulsed suction to the gutter rather than steady, continuous suction. This is stated to reduce the amount of solvent lost from the ink, because of the reduction in the amount of air sucked into the ink system from the gutter. It also proposes that the mixture of ink and air passing from the gutter to the ink tank or alternatively the air being discharged from the ink tank may be cooled or otherwise treated to reduce the level of solvent droplets and/or vapour discharged to the environment.
EP-A-0076914 proposes that the vacuum source should apply only a very low level of suction (e.g. about ten centimeters of water) to the gutter, in order to minimise the flow of air along the gutter line and thereby reduce the rate of evaporation of solvent from the ink. It additionally proposes that the ink should be cooled before it is supplied to the ink gun, in order to reduce the rate of evaporation at the printhead.
Proposals to cool the mixture of ink and air flowing from the gutter, or to cool the air before it is discharged to atmosphere, in order to condense solvent out of it are also disclosed in JP-01-247167, EP-A-0805038, U.S. Pat. No. 5,532,720, WO93/17868, WO93/17869 and WO94/07699.
Condensation of solvent vapour from vented air is used in practice in the A200, A300 and A400 ink jet printers available from Domino UK Limited, Trafalgar Way, Bar Hill, Cambridge CB3 8TU, which optionally include a Peltier device arranged to cool air flowing out of the ink tank so as to condense solvent vapour in the air. The condensed solvent is discharged to the solvent tank and the air is vented. This reduces the rate at which the printer consumes solvent.
The reduction of solvent consumption is useful, partly because solvent consumption represents a significant cost in the running of a continuous ink jet printer, and also because (as will be clear from the examples given above) the solvents tend to be volatile organic compounds and therefore solvent discharge to the atmosphere is environmentally disadvantageous. However, it needs to be borne in mind in the design of any arrangement for recovering evaporated solvent by condensation that excessive cooling of solvent-laden air will tend to cause water to condense in addition to solvent, and the introduction of water into the ink or solvent is highly undesirable in most continuous ink jet printer ink compositions.
U.S. Pat. No. 4,283,730 and U.S. Pat. No. 4,356,500 propose a system in which the air which has passed down the gutter line is returned to the space enclosed by the printhead cover, so that the air within the printhead cover becomes substantially saturated with solvent. This is intended to prevent ink from evaporating from the ink jet while it is in the space enclosed by the cover, so as to reduce solvent consumption, and also to prevent ink splashes at the printhead from drying. It proposes that, if the ink jet is cooler than the air within the printhead cover, there may be recondensation of solvent into the ink jet. It also proposes that electrodes may be heated slightly to prevent solvent from condensing on them. However, the present inventors consider that in many ink jet printer designs it is desirable for ink splashes to dry as quickly as possible, rather than to be prevented from drying, because the electrically conductive nature of wet ink tends to interfere with the correct functioning of printhead electrodes. It may be noted that U.S. Pat. No. 4,283,730 and U.S. Pat. No. 4,356,500 relate to an uncommon printhead design in which ink drops make grazing contact with a curved surface and then drops to be printed separate from the surface again under centrifugal force.
U.S. Pat. No. 4,184,167 concerns a continuous ink jet printer in which the gutter is provided by a knife-edge at the end of one of the electrodes used to create the deflection field. The surface of the electrode is porous stainless steel and the ink is sucked through it by a vacuum pump. The air which is also sucked through the electrode becomes laden with solvent and is then delivered to the other electrode used to create the deflection field. The solvent laden air passes through the porous stainless steel face of this electrode to provide a barrier to prevent stray ink drops from adhering to and drying on the surface of that electrode, and also prevents the drying of ink drops which have contacted the surface of the first electrode before reaching the gutter-forming knife-edge, so that the drops remain liquid and are sucked through the electrode by the vacuum source.
EP-A-0560332 proposes that air which has passed from the gutter into the ink tank and is then vented from the ink tank should be cooled, to recover some of the vaporised solvent, and then the air is returned to the printhead outside the gutter. Accordingly the air which is sucked into the gutter is air which has previously passed through the gutter, the ink tank and the cooler before being returned to the printhead. Consequently, the same air circulates continuously within the printer. Since air does not flow out of the printer, solvent loss is substantially prevented.
WO93/17869 also proposes that air vented from the ink tank may, after being cooled to recover vaporised solvent, be vented at the printhead adjacent the ink nozzles so that residual solvent vapour remaining in the air is carried with the stream of ink droplets and sucked into the gutter so as to minimise the escape of solvent vapour into the environment.
Although these arrangements for returning air which has entered the gutter back to the printhead are, in theory, effective for reducing solvent loss, in practice they will tend to result in the condensation of solvent on electrodes and other parts of the printhead unless steps are taken to avoid this such as heating the electrodes and other parts as proposed in U.S. Pat. No. 4,283,730 and U.S. Pat. No. 4,356,500 or removing some of the solvent vapour from the air as proposed in EP 0560332 and WO93/17869 with result that the air returned to the printhead is not fully saturated.
Because the ink is normally electrically conductive when wet, and is controlled by being given an electric charge and steered by electric fields, condensation of solvent on parts of the printhead can disrupt the electrical deflection operation, either by distorting the shape of electrical fields or by shorting electrodes, or may interfere in other electrical operations such as electrically sensing charged drops during jet speed measurement or other control operations.